Plasma-spray burner and process for operating the same

ABSTRACT

A plasma-spray burner and process for operating the same includes a longitudinally reciprocable cathode disposed upstream of a nozzle which serves as an anode, a power source being electrically connected therebetween. The cathode is movable between at least two positions one of the positions serving to seal the inlet to the nozzle whereby the flow of operational gas through the nozzle is terminated. Means are also provided for synchronously controlling the movement of the cathode in conjunction with the supply of power to the cathode and anode in a pulsed fashion whereby power will be supplied when the cathode is moved to a position which permits the flow of gas to occur through the nozzle, an arc thereby existing between the cathode and anode, and wherein such power will be terminated when the cathode is moved to the position which seals the nozzle inlet.

119-12lo SR United States Patent 1 [111 3,803,380 Ragaller Apr. 9, 1974 PLASMA-SPRAY BURNER AND PROCESS Primary Examiner-Bruce A. Reynolds FOR OPERATING THE SAME Attorney, Agent, or Firm-Oblon, Fisher, Spivak, Mc-

[75] inventor: Klaus Ragaller, Neuerihof, Clenand & Maler Switzerland [73] Assignee: BBC Brown Boveri & Company [57] ABSTRACT Limited, Baden, Switzerland A plasma-spray burner and process for operating the [22] Filed: Man 12, 1973 same includes a longitudinally reciprocable cathode disposed upstream of a nozzle which serves as an an- PP Q' 340,533 ode, a power source being electrically connected therebetween. The cathode is movable between at least two positions one of the positions serving to seal 30 F'Al't"P"tDt l 1 orelgn pp y a a the inlet to the nozzle whereby the flow of operational Mar. 16, 1972 Switzerland 3928/72 g through the nozzle is terminated. Means are also provided for synchronously controlling the movement (g! 2l9/l2lBP2,32kl of the cathode in conjunction with the supply of [58] 77 121 P powerto the cathode and anode in a pulsed fashion 0 care whereby power will be supplied when 'the'cathode is I moved to a position which permits the flow of gas to [56] References cued occur through the nozzle, an are thereby existing be- UNITED STATES PATENTS tween the cathode and anode, and wherein such 3,524,956 8/1970 Rocklin' 219/76 power will be terminated when the cathode is moved 3,004,189 Giannini P to the position seals the nozzle inlet 3,324,278 6/1967 Jackson 219/74 X 6 Claims, 2 Drawing Figures I l l PATENTEIJAPR 91914 SHEUHFZ 85803380 10 11 4 I E 8th g l )3 9 a 2 Fig. 1

PLASMA-SPRAY BURNER AND PROCESS FOR OPERATING THE SAME BACKGROUND OF THE INVENTION The present invention relates generally to burners and more particularly to plasma-spray burners and a process for operating the same in which an arc exists between a cathode and a nozzle, serving as the anode and through which flows the operational gas, the are serving to generate the plasma jet in the form of particles to be sprayed.

Plasma-spray burners, that is, plasma-burners for the deposition of surface layers, particularly with respect to substances having high melting points and which are generally difficult to deposit, are of course known, such as for example, as disclosed in ZEITSCl-IRIFT FUER WIRTSCI-IAFTLICHE FERTIGUNG 64 1969, Heft 6, 277-282; TECHNICA 1968 No. 19, Pages 1671-1720. Such burners most often involve DC plasma burners provided with a tungsten cathode and a copper nozzle acting as anode, the plasma jet being generated by means of an inert gas and wherein the burner is operated under a power of several tens of kw. The substance to be sprayed is introduced into the plasma jet in the form of particles ranging in size from to 100 microns. These particles may be suspended within a gas stream and thus be mixed with the plasma jet, or they may be introduced as a result of melting wires of the substance to be deposited. The particles are heated within the plasma jet, preferably to .a temperature where they will be plastic or liquid and are accelerated by the rapid plasma flow. If the particles then impinge upon the substrate to be coated, a strongly adherent and hermetic layer will be formed.

The known plasma-spray burners however suffer from several drawbacks, one of which is the fact that only particles with speeds ranging from about 100-300 m/sec can be accelerated. Therefore, the sprayed-on layers will often not be of the desired thickness or attain the desired adhesive properties. Furthermore, the operation of known plasma-spray burners is substantially uneconomical due to the fact that a large part of the ordinarily expensive operational gas, which is ineffective with respect to the spraying process, will often flow out, in a cooling fashion, between the arc and the nozzle wall. Finally, oxidizing and corrosive gases, which would of course be desirable with respect to the cost factor, may not be used due to the fact that the tungsten cathode would burn in an oxidizing medium at high temperature. I

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved plasma-spray burner and a method of operating the same.

Another object of the present invention is to provide an improved plasma-spray burner which is capable of coating an article with a layer having a desired, uniform thickness and which exhibits desirable adhesive properties.

Still another object of the present invention is to provide an improved plasma-spray burner which exhibits a low percentage loss of the operational gas and spray substance.

Yet another object of the present invention is to provide an improved plasma-spray burner which is economical to operate due to the fact that cheap gases, including corrosive gases, may be utilized without jeopardizing the structural integrity of the apparatus.

The foregoing objectives are achieved according to this invention through the provision of aplasma-spray burner in which operational gas is subjected to a pressure of 2 atmospheres prior to its entry into the nozzle, utilizing an arc-burning current of at least 1,000 amperes within the system of the invention to attain the type of plasma-spray process initially indicated, and by synchronously pulsing the gas flow through the nozzle with the arc-burning current. Preferably, the gas flow through the nozzle and the arc-burning current will be substantially zero between the pulses. In addition, the pressure of the operational gas prior to its entry into the nozzle is very high, for instance 20 atmospheres, and the value of the burning current shall also be as high as possible, such as for example, 20 kamp.

These measures rest upon the assumptions, confirmed by measurements and theory, that speeds of several 1,000 m/sec may be achieved in plasma jets provided that large arc-currents and gas pressures are present. Such high current and pressure values however load the nozzle and the cathode, in particular, within conventional burners that considerable expenditures are required for cooling, rendering such operations unfeasible and highly uneconomical. The high degrees of pressure and current may only be applied when making use of synchronous pulses of pressure and current, since for such pulse-like loading of the nozzle and cathode, followed by freedom from such loading, the expenditures for cooling will not exceed conventional bounds.

High speeds of the plasma jet thus being imparted to the particles to be sprayed, an extraordinarily adhesive and hermetic surface layer will be achieved. The cathode will be loaded only minimally and therefore oxidizing gases, if compatible with the material to be sprayed, such as air, which is most economical, may be used. As regards the selection of the operational gas and the substance to be sprayed, there no longer remain practical restrictions as set by the burner.

BRIEF DESCRIPTION OF THE DRAWINGS Various other objects, features and attendant advantages'of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic embodiment of a burner constructed according to the present invention; and

FIG. 2 is a graph showing the time-variation of the position of the cathode, the current and the ignition pulse of the burner of FIG. 1. I

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings, and more particularly to FIG. 1 thereof, the burner of the present invention includes a cylindrical cathode which is capable of being shifted in an axial direction and which is preferably made of tungsten, and a Laval nozzle 2, which is preferably made' of copper and which is located downstream of cathode l. The nozzle is surrounded by a dish 3, which may be made of brass, and which is configured so as to provide an annular channel 4 in which a coolant may circulate for cooling nozzle 2. The coolant,

which may be water, is fed to and removed from channel 4, along a path indicated by the arrows shown, through radially extending passages.

An insulator 5 is mounted upon the top of nozzle 2 and is provided with a radial inlet 6 which serves to admit the operational gas under high pressure such as for example, 20 atm., together with the particulate spraying substance, both substances being further transmitted to a combustion chamber. At the upper portion of insulator 5 there is provided an axial bore 7 in conjunction with which there is mounted an O-ring type seal, through which passes the displaceable cathode 1.

A spring 9, disposed about the upper portion of cathode 1 and between a flanged portion of cathode l and a stop 8, serves to bias the cathode 1 toward the inlet of nozzle 2 and seal the latter unless cathode 1 is raised by means of a fulcrummed lever 10 pivotally connected to the upper portion of cathode 1 and actuated by means of a cam 11. A D. C. power source 12 is also provided and the current may be switched on or off by means of a thyristor adjustment component 13 which is also synchronously controlled with lever 10 by means of cam 11, the position of the components shown in the drawing illustrating the ON mode of operation. In this mode, an are thus exists between cathode l and nozzle 2, which serves as the anode, causing the operational gas located above the nozzle inlet to flow out of the nozzle 2 as a luminous plasma jet 14 at supersonic speed. Plasma jet 14 then impacts upon a substrate 15 so that the latter will be coated by means of the sprayparticles which were introduced into the operational gas via inlet 6.

The burner of FIG. 1 is of course shown in schematic form, various parts thereof obviously lending themselves to different modifications. As a rule, for example, the cathode 1 will be provided with coolant means in addition to the coolant provided for anode 2. In addition, the displacement mechanism for cathode 1 may not be, as shown in the embodiment, the cam-lever mechanism 10 and 11, but on the contray, such may be in the form of an electro-magnet for cathode 1, which will render the burner easier to operate.

The burner as shown is operated such that a high pressure, for example, preferably atm. and at least 2 atm., exists within the combustion chamber and that the current intensity when switched to the ON mode amounts to several kamp., such as for example, preferably 20 kamp, and at least 1 kamp. An arc will then exist when the relative position between the cathode and the anode is as shown, the potential therebetween being of the order of 1 kv., and the plasma jet 14 will exit from nozzle 2 with a speed of several l,000 m/sec. It is important that the nozzle intake have a small radius of curvature, preferably less than 15 mm., so as to assure are stability.

The cam-lever mechanism 10 and 11 will of course be moved longitudinally up and down in order to achieve pulsing of the gas stream, cathode 1 thus being moved up and down in accordance with the movement of lever 10, whereby the nozzle inlet will be opened and closed in a pulse-like manner. Referring to FIG. 2, the path S(t) of cathode 1, plotted as a function of time t, is shown. Once cathode 1 has been moved so as to open the nozzle inlet, a high potential pulse will be applied to the cathode and anode by means of a known device, not shown, and the arc will be ignited. This pulse will initiate a current l(t), which is a function of time, via the thyristor control component 13, and the arc ignited between cathode l and anode 2 is immediately propelled into nozzle 2 by means of the intense gas stream. The ignition pulse Z(t), which is also a function of time, for thyristor control component 13, and the pertinent synchronous high potential pulses are of course determined by the position of cam 11.

While the powder to be sprayed upon substrate 15 may be added to the gas within the high pressure compartment via inlet 6, such may alternatively be admitted to the plasma jet at any location within the nozzle 2.

In order to prevent the loss of any operational gas, one must attempt to fill the nozzle with the arc to an extent which is as great as possible, and in such case, no cooling gas will be wasted between the arc and nozzle. This is feasible if the arc current strength is maintained at a value approxirriating 20 kamp/cm2 at the 555mest nozzle cross-section, such magnitude being obtained from an empirical estimate. In connection with the last-mentioned mode of operation, thermal loading of the nozzle 2 is so large that the pulse duration T should not exceed 5-10 msec. Subsequently, one must observe a pulse interval T of, for example, msec. until there is sufficient cooling by means of the coolant.

In addition to the use of economical operational gas, such as for example, air, other clear gases, such as for example, argon and the like, may also be used. This will be appropriate if the spray substance is made of a substance which is not compatible with oxidizing gases, such as is in the case of copper. By means of the burner, one may thus coat steel with tungsten carbide, copper with A1 0 or brass withtungstenfMai'kedly corrosive gases may also be used, the burner not being jeopardized due to the operation under conditions exhibiting pulse-like heating. The losses as regards the operational gas and spray substance are appreciably less for the burner of the present invention than they are for conventional plasma spray burners.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood therefore that within the scope of the appended claims the present invention may be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by letters patent of the United States is:

1. A process for operating a plasma-spray burner in which an arc exists between a cathode and a nozzle acting as anode and transmitting an operational gas, the are serving to generate the plasma jet containing the material to be sprayed in particulate form, comprising the steps of:

subjecting the operational gas to a pressure exceeding two atmospheres prior to entering said nozzle;

providing an arc-buming current which exceeds one thousand amperes; and pulsing said gas flow through said nozzle and synchronously pulsing said arc-buming current through said nozzle.

2. A process as set forth in claim 1, wherein said gas flow through said nozzle and said arc-buming current will be substantially zero between said pulses.

3. A proces as set forth in claim 1, wherein said pressure impressed upon said operational gas will be equal 5. A process as set forth in claim 1, wherein said operational gas being utilized is air when spraying said materials which are insensitive to oxidation.

6. A process as set forth in claim 1, wherein said material to be sprayed is in particulate form, said particles ranging in size from ll) to microns, and is added to said operational gas prior to said entrance of said gas into said nozzle. 

1. A process for operating a plasma-spray burner in which an arc exists between a cathode and a nozzle acting as anode and transmitting an operational gas, the arc serving to generate the plasma jet containing the material to be sprayed in particulate form, comprising the steps of: subjecting the operational gas to a pressure exceeding two atmospheres prior to entering said nozzle; providing an arc-burning current which exceeds one thousand amperes; and pulsing said gas flow through said nozzle and synchronously pulsing said arc-burning current through said nozzle.
 2. A process as set forth in claim 1, wherein said gas flow through said nozzle and said arc-burning current will be substantially zero between said pulses.
 3. A process as set forth in claim 1, wherein said pressure impressed upon said operational gas will be equal to or greater than 2 atm. and less than or equal to 20 atm. and said arc-burning current will be equal to or greater than 1 kamp and less than or equal to 20 kamp. during a pulse.
 4. A process as set forth in claim 2, wherein for an operational pressure of 20 atm. and an arc-burning current of 20 kamp., said pulse duration encompassing said gas flow and arc discharge is less than 10 msec and said duration of time wherein there is no gas flow and arc discharge is greater than 100 msec.
 5. A process as set forth in claim 1, wherein said operational gas being utilized is air when spraying said materials which are insensitive to oxidation.
 6. A process as set forth in claim 1, wherein said material to be sprayed is in particulate form, said particles ranging in size from 10 to 100 microns, and is added to said operational gas prior to said entrance of said gas into said nozzle. 